remove unnecessary __restrict__ modifiers

__restrict__ in a pointer returned from a function is without effect according to c99 (assuming gnu's __restrict__ implements c99), see https://en.cppreference.com/w/c/language/restrict.html, 3rd paragraph. And for function parameters, it's only needed in definitions not in the declarations, see https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html.